The invention relates to vehicles for industrial and agricultural use, such as utility tractors. Particularly, the invention relates to engine anti-stall transmission control of a utility vehicle that incorporates a hydrostatic transmission as an operator-controlled speed-adjusting component of the vehicle drive train.
Compact utility tractors having hydrostatic transmissions are commonly purchased by customers that are not experienced tractor operators and are used for jobs, such as material handling with a front loader, that subject the tractor to sudden load application. With inexperienced drivers, engine stall may occur, leading to operator frustration and a perception that the tractor lacks sufficient engine power. This problem is exacerbated with foot pedal control of the hydrostatic transmission because in order to prevent engine stalling, the operator must actually reduce the pedal actuation, to decrease the stroke of the hydrostatic pump. This however is contrary to the action usually applied to foot pedal accelerators in cars and trucks to prevent engine stalling under load.
The invention provides an apparatus and method to prevent engine stalling, in a utility vehicle having a hydrostatic transmission, by automatically reducing the stroke of the hydrostatic transmission pump, and thus the vehicle speed, when engine speed drops below a predetermined threshold.
The apparatus and methods of the invention effectively prevent engine stall in a tractor having hydrostatic transmission speed control. The apparatus and methods can prevent engine stall when the vehicle is operating in either forward or reverse direction.
The apparatus includes a hydrostatic transmission, an engine speed sensor, a throttle position sensor and a controller, such as a microcontroller. A servo piston operating against a centering spring is moved to adjust the capacity of the pump in the hydrostatic transmission. The position of the servo piston is controlled by electro-hydraulic proportional pressure reducing valves that modulate the pressure applied to the servo piston. In normal vehicle operation, the operator depresses a foot pedal. A potentiometer senses the position of the foot pedal and sends a voltage signal to the controller. The controller software calculates a command current from the signal and drives the pressure reducing valves with the current. The greater the current, the greater the pump stroke and the faster the tractor wheels are turned.
As the tractor comes under load, the engine speed begins to drop. The microcontroller software continuously monitors the engine speed from a pulse pickup unit, and compares the engine speed to the estimated unloaded engine speed which is based on the position of the throttle lever as measured by the throttle position sensor. The engine speed is allowed to drop an amount specified by a software parameter. When the engine speed drop exceeds a threshold, then the microcontroller responds by reducing the current command to the pressure reducing valves, thus reducing the pump capacity and the tractor wheel speed. The amount of current reduction is calculated using a PID algorithm using the error between the unloaded speed, estimated from the throttle position sensor, and the actual engine speed.
As a further aspect of the invention, a method is provided for reducing the cost of implementing engine stall prevention in utility vehicles. The inventive system eliminates the need for a throttle position sensor, thus reducing the overall cost of the system.
The further aspect of the invention establishes a method for predicting the unloaded engine speed for anti-stall control from a measurement of engine speed while the transmission controls of the vehicle are in a neutral position.
The further aspect of the invention uses an engine speed sensor such as a pulse pickup unit, speed control foot pedals or other manual direction control, and a microcontroller with software. The software monitors the engine speed and the foot pedal speed controls continuously. When the foot pedals are in neutral, i.e., neither the forward or the reverse pedal is depressed, and the vehicle is not moving, the software captures and stores the engine speed in a microcontroller memory. This is an accurate estimate of the unloaded engine speed. As the operator commands the vehicle into motion, the engine speed will drop depending on the level of load. When the engine speed drop exceeds a specified value compared to the unloaded engine speed, a control command is sent from the microcontroller to the transmission to reduce the vehicle speed proportional to the amount of engine drop, and thus the power required. When the driver changes direction via foot pedal, the engine speed recovers rapidly, thus permitting an updated measurement of unloaded engine speed to be captured by the microcontroller memory.
This process works best when the engine throttle position is left unchanged during vehicle operation, but does work successfully if the operator changes throttle position while the vehicle is not moving.
This further aspect of the invention is particularly applicable to vehicle operations that involve regular changes in direction, such as moving materials with a front-end loader. Because the driver regularly shifts the transmission through neutral when changing direction, the microcontroller can regularly and accurately update its measurement of the unloaded engine speed without the need for a throttle position sensor. This further aspect of the invention permits engine stall prevention to be implemented at a reduced cost.
Numerous other advantages and features of the present invention will become readily apparent from the following detailed description of the invention and the embodiments thereof, from the claims and from the accompanying drawings.